Although estimates of heritability of performance in the Thoroughbred racehorse have been calculated to be between 10-40%, no genes influencing performance have been identified. Based on comparative studies with human, we identified the angiotensin-converting enzyme (ACE) gene as an equine candidate gene for performance. From characterization of nine SNPs, nine equine ACE haplotypes were identified in 34 horses. The haplotypes occurred at differing frequencies in breeds of different athletic capabilities, including endurance Arabians, Thoroughbred racehorses and Draught horses. One haplotype had a similar effect on equine ACE levels as the recognised Alu insertion/deletion polymorphism in the human gene. Further studies are suggested to test the association of this haplotype with a range of indices of performance in substantially larger cohorts of horses before these markers can be used for prediction of racing performance.

A comprehensive male linkage map was generated by adding 359 new, informative microsatellites to the International Equine Gene Map half-sibling reference families and by combining genotype data from three independent mapping resources: a full sibling family created at the Animal Health Trust in Newmarket, United Kingdom, eight half-sibling families from Sweden and two half-sibling families from the University of California, Davis. Because the combined data were derived primarily from half-sibling families, only autosomal markers were analyzed. The map was constructed from a total of 766 markers distributed on the 31 equine chromosomes. It has a higher marker density than that of previously reported maps, with 626 markers linearly ordered and 140 other markers assigned to a chromosomal region. Fifty-nine markers (7%) failed to meet the criteria for statistical evidence of linkage and remain unassigned. The map spans 3,740 cM with an average distance of 6.3 cM between markers. Fifty-five percent of the intervals are < or = 5 cM and only 3% > or = 20 cM. The present map demonstrates the cohesiveness of the different data sets and provides a single resource for genome scan analyses and integration with the radiation hybrid map.

A low-density, male-based linkage map was constructed as one of the objectives of the International Equine Gene Mapping Workshop. Here we report the second generation map based on testing 503 half-sibling offspring from 13 sire families for 344 informative markers using the crimap program. The multipoint linkage analysis localized 310 markers (90%) with 257 markers being linearly ordered. The map included 34 linkage groups representing all 31 autosomes and spanning 2262 cM with an average interval between loci of 10.1 cM. This map is a milestone in that it is the first map with linkage groups assigned to each of the 31 automosomes and a single linkage group to all but three chromosomes.